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Have you considered maybe going to a GM RamJet 350. It has good HP, and is already port fuel injected .... so nearly plug & play . Plus it looks pretty cool for a crate motor, kind of has that old mech. fuel injection look to it. Match that up to a 700R4 and your in great shape.
By the way, making the old mid 70's engine produce more HP by making them more efficient also result in better MPG. I have a ZZ4 modified for extra power, and I still get 19-20mpg on the highway with NO overdrive. That's better than when I bought my 76 with the old L48 engine in it from the factory.